With a stupidly over complex vehicle and gearbox there are many different failure modes so telling you what is wrong is very difficult...
Have you confirmed you have a good voltage level yet? As without that you are just ****ing in the wind.
If it rumbles or vibrates between 40-50, the TQ is on the way out and on the petrol models this is caused by a failed O ring that seals the B and C clutches and causes reduced hydraulic pressure holding A cluth in, this causes it to slip which the TQ tries to control which leads it to judder and vibrate...eventually wearing out the TQ friction material....
That is just one of the most common failures of the box...
There are many many more. The L322 requires patience and very deep pockets to run and maintain.
Not engaging 5th gear could be a sign the F clutch (I think that is the one) is failing and slipping....
In Sport mode the TQ is commanded to slip to maintain maximum torque from the engine as locking up can bog down the engine hampering performance....
So if your box doesn't engage 5th in normal mode this could mean 5th is slipping to much for the TQ to be able to lock up correctly....whereas in Sport mode the TQ doesn't need to lock up and hence why it will engage...
This is my simplistic view on it....Bemble is the man to ask as he knows these boxes like the back of his hand....
Have you checked fluid level and condition? Filter condition?
But first....sort out the suspect voltage issues!